Method and apparatus for modulating a pulse signal with a bit stream

ABSTRACT

A method and apparatus for modulating a pulse signal with a bit stream is disclosed. The pulse signal is modulated by selectively inverting and delaying signal pulses within the pulse signal responsive to bits within the bit stream.

RELATED APPLICATION

This application claims the benefit of the filing date of provisionalapplication No. 60/450,313 entitled “BIORTHOGONAL MODULATION FORSUPPRESSING ULTRA-WIDEBAND SPECTRAL LINES” filed Feb. 27, 2003, thecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to communication systems and, moreparticularly, to methods and apparatus for modulating a pulse signalwith a bit stream.

BACKGROUND

Ultra Wideband (UWB) technology, which uses base-band pulses of veryshort duration to spread the energy of transmitted signals very thinlyfrom near zero to several GHz, is presently in use in militaryapplications. Commercial applications will soon become possible due to arecent Federal Communications Commission (FCC) decision that permits themarketing and operation of consumer products incorporating UWBtechnology.

The key motivation for the FCC's decision to allow commercialapplications is that no new communication spectrum is required for UWBtransmissions because, when they are properly configured, UWB signalscan coexist with other application signals in the same spectrum withnegligible mutual interference. In order to ensure negligible mutualinterference, however, the FCC has specified emission limits for the UWBapplications. For example, a basic FCC requirement is that UWB systemsdo not generate signals that interfere with other narrowbandcommunication systems.

The emission profile of a UWB signal can be determined by examining itspower spectral density (PSD). The PSD for ideal synchronous data pulsestreams based upon stochastic theory is well known and is described inan article by M. Z. Win, entitled “Spectral Density of RandomTime-Hopping Spread-Spectrum UWB Signals with Uniform Timing Jitter”,Proc. MICOM'99, vol. 2, pp. 1196–1200, 1999. This article also providesa characterization of the PSD of the Time-Hopping Spread Spectrumsignaling scheme in the presence of random timing jitter using astochastic approach.

The power spectra of UWB signals consist of continuous and discretecomponents. Generally speaking, discrete components contribute more tothe PSD than continuous components, which behave as white noise. Thus,discrete components cause more interference to narrowband wirelesssystems than continuous components. Accordingly, a basic objective inthe design of UWB systems is to reduce the discrete component of the UWBpower spectra. Another objective for UWB systems is to increase thepower efficiency.

UWB communication system currently use one of two modulation techniques.These techniques include a pulse position modulation (PPM) technique anda bi-phase shift keying (BPSK) technique. The PPM technique has goodpower efficiency but a relatively high PSD. The BPSK technique, on theother hand, has a relatively low PSD but low power efficiency.

There is an ever present desire for efficient communication systems thattransmit signals with low PSD. Accordingly, there is a need for improvedmodulation methods, apparatus, and systems that are not subject to theabove limitations. The present invention fulfils this need among others.

SUMMARY OF THE INVENTION

The present invention is embodied in a method and an apparatus thatmodulates a pulse signal with a bit stream. The pulse signal ismodulated by selectively inverting and delaying signal pulses within thepulse signal responsive to bits within the bit stream.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawings, with likeelements having the same reference numerals. Included in the drawingsare the following figures:

FIG. 1 is a block diagram of a transmitter for modulating a pulse signalwith a bit stream in accordance with the present invention.

FIGS. 1A, 1B and 1C are block diagrams of alternative exemplary pulsegenerators in accordance with the present invention for use in thetransmitter of FIG. 1.

FIG. 2 is a flow chart of exemplary steps for modulating a pulse signalwith a bit stream in accordance with the present invention.

FIG. 3 is a graph of amplitude versus frequency that shows thepower-spectral density (PSD) of a single monocycle pulse.

FIG. 4 is a graph of amplitude versus frequency that shows the PSD of apulse-position modulated ultra-wideband (UWB) signal in accordance withprior art;

FIGS. 5A and 5B are graphs of amplitude versus time which illustrate amonocycle pulse and an inverted monocycle pulse, respectively, inaccordance with prior art.

FIG. 6 is a graph of amplitude versus frequency that shows the PSD of abi-phase modulated UWB signal in accordance with prior art.

FIGS. 7A, 7B, 7C and 7D are graphs of amplitude versus time whichillustrate a monocycle pulse, an inverted monocycle pulse, a delayedmonocycle pulse, and an inverted and delayed monocycle pulse,respectively, in accordance with the present invention.

FIG. 8 is a graph of amplitude versus frequency that shows the PSD of apulse signal in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts select components of an exemplary transmitter 100 formodulating a pulse stream with a bit stream for transmission inaccordance with the present invention. A controller 102 includes a phasecontroller 112 and a delay controller 114. In the illustratedembodiment, the phase controller 112 and the delay controller 114 areeach coupled to the pulse generator 104 and to a shift register 110. Thephase controller 112 is coupled to a first position 110 a of the shiftregister (e.g., to receive a first bit of the bit stream) and the delaycontroller 114 is coupled to a second position 110 b of the shiftregister (e.g., to receive a second bit of the bit stream). It will berecognized by those of skill in the art that the designation of firstand second positions 110 a and 110 b is for ease of description and notto suggest a particular bit processing order. For example, the phasecontroller 112 and delay controller 114 may be coupled to the secondposition 110 b and the first position 110 a, respectively. Also, thefirst and second register positions referred to with numerals “110 a”and “110 b” may be interchanged.

In an exemplary embodiment, the shift register 110 is a shift registerthat shifts two bits of the bit stream during each shift such that thecontroller 102 receives two new bits of the bit stream for processing ateach shift. In an alternative exemplary embodiment, the shift registershifts one bit at a time and the controller 102 is configured to passthe bits to the appropriate phase/delay controller 112/114. Variousother embodiments will be apparent to those of skill in the art from thedescription herein. The phase/delay controllers 112/114 may each be alatch that produces the value of the received bit at an output port.

A pulse generator 104 generates a pulse signal that includes a pluralityof signal pulses. The pulse generator 104 is coupled to the phasecontroller 112 and the delay controller 114 to receive a phase signaland a delay signal, respectively, therefrom. In an exemplary embodiment,the pulse generator is a monocycle pulse signal generator that generatesmonocycle signal pulses such as a UWB pulse generator that generates UWBsignal pulses.

The pulse generator 104 alters the signal pulses within the pulse signalresponsive to the phase signal and the delay signal received,respectively, from the phase controller 112 and the delay controller114. In an exemplary embodiment, for a monocycle pulse signal, the pulsegenerator 104 selectively inverts the signal pulses responsive to thephase signal and selectively delays the signal pulse by a predefinedamount responsive to the delay signal. Thus, the pulse generator 104 mayalter the pulse signal by producing signal pulses with no delay orinversion, a delay, an inversion, or a delay and an inversion. In anexemplary embodiment, the signal pulse is delayed by an amountsufficient to substantially decorrelate a delayed signal pulse from thesignal pulse prior to delay. For example, the signal pulses may bedelayed such that a delayed signal pulse is orthogonal to the signalpulse prior to delay. The altering of the signal pulses is described infurther detail below.

FIG. 1A depicts an exemplary pulse generator 150 for use as the pulsegenerator 104 (FIG. 1). The alternative exemplary pulse generator 150includes a plurality of pulse generators 152 that each generate a signalpulse. A selector 154 is coupled to the plurality of pulse generators152, the phase controller 112 (FIG. 1), and the delay controller 114(FIG. 1). The selector 154 is configured to select a signal pulsegenerated by a particular one of the plurality of pulse generators 152responsive to phase and delay signals from the phase controller 112 andthe delay controller 114, respectively. The illustrated plurality ofpulse generators 152 include a first pulse generator 156, a second pulsegenerator 158, a third pulse generator 160, and a fourth pulse generator162. The first pulse generator 156 may be configured to generate a firstsignal pulse without either delay or inversion. The second pulsegenerator 158 may be configured to generate a second signal pulse thatis inverted but not delayed with respect to the first signal pulse. Thethird signal pulse generator 160 may be configured to generate a thirdsignal pulse that is delayed by not inverted with respect to the firstsignal pulse. The fourth pulse generator 158 may be configured togenerate a fourth signal pulse that is both delayed and inverted withrespect to the first signal pulse. By selecting one of the first throughfourth signal pulses responsive to the phase and delay signals, theselector 154 is able to produce signal pulses that are selectivelyinverted and delayed.

FIG. 1B depicts an alternative exemplary pulse generator 170 for use asthe pulse generator 104 (FIG. 1). The alternative exemplary pulsegenerator 170 includes a pair of pulse generators 172 that each generatea signal pulse. A selector 174 is coupled to the pair of pulsegenerators 172 and the phase controller 112 (FIG. 1). A delay circuit176 is coupled to the selector 174 and the delay controller 114 (FIG.1). The selector 174 is configured to select a signal pulse generated bya particular one of the pair of pulse generators 172 responsive to phasesignals from the phase controller 112 and the delay circuit 176 isconfigured to selectively introduce delay responsive to delay signalsfrom the delay controller 114. The illustrated pair of pulse generators172 include a first pulse generator 178 and a second pulse generator180. The first pulse generator 178 may be configured to generate a firstsignal pulse without inversion. The second pulse generator 180 may beconfigured to generate a second signal pulse that is inverted withrespect to the first signal pulse. By selecting one of the first andsecond signal pulses responsive to the phase signal and selectivelyintroducing delay to the selected signal pulse responsive to the delaysignal, the selector 174 and delay circuit 176 are able to producesignal pulses that are selectively inverted and delayed. It will beunderstood by those skilled in the art that the delay circuit 176 may bepositioned to introduce delay to the signal pulses generated by the pairof pulse generators 172 prior to selection of a particular signal by theselector 174. Various alternative arrangements will be understood bythose of skill in the art from the above description.

FIG. 1C depicts an alternative exemplary pulse generator 190 for use asthe pulse generator 104 (FIG. 1). The alternative exemplary pulsegenerator 190 includes a single pulse generator 192 that generates asignal pulse. An inverter 194 is coupled to the pulse generator 192 andthe phase controller 112 (FIG. 1). A delay circuit 196 is coupled to theinverter 194 and the delay controller 114 (FIG. 1). The inverter 194 isconfigured to selectively invert the signal pulse responsive to thephase signal from the phase controller 112 and the delay circuit isconfigured to selectively delay the signal pulse responsive to the delaysignal from the delay controller 114 to produce signal pulses that areselectively inverted and delayed. It will be understood by those skilledin the art that the delay circuit 196 may be positioned to introducedelay to the signal pulse generated by the pulse generator 192 prior toselective inversion by the inverter 194. Various alternativearrangements will be understood by those of skill in the art from theabove description.

Referring back to FIG. 1, an optional time-hopping controller 106introduces time-hopping to the pulse signal to position each signalpulse at a different time-hop index inside a frame. In the illustratedembodiment, the time-hopping controller 106 is coupled to the pulsegenerator 104. In an exemplary embodiment, the time-hopping controller106 introduces time-hopping to the pulse signal selectively inverted anddelayed 102 in a conventional manner.

The antenna 108 transmits the pulse signal. In the illustratedembodiment, the antenna 108 is coupled to the time-hopping controller106. In this embodiment, the antenna 108 transmits a pulse signal asaltered according to the pulse controller 102 and time-hopped by thetime-hopping controller 106. In embodiments where the pulse signal isnot time-hopped, the antenna 108 is coupled to the pulse generator 104for transmitting the pulse signal without time-hopping. The pulse signalas selectively inverted and delayed and, optionally, as time-hopped istransmitter via the antenna 108.

A receiver (not shown) receives the pulse signal from the transmitter100. In an exemplary embodiment, the receiver uses a predefined pulsetemplate to correlate incoming signals and, then, performs anintegration over the pulse template. The template shifts forward andbackward to find peaks of the integration. The position of the peak isused to identify the position of pulses in the PPM and the polarity ofthe peaks is used to determine phase (e.g., positive maximum valueindicates normal phase and negative maximum value indicates invertedphase). If the optional time-hopping controller 106 is used in thetransmitter 100, it is desirable for the receiver to employ acomplementary time-hopping controller (not shown) having the sametime-hopping sequence used by the time-hopping controller 106 to locateeach of the transmitted pulses in each frame so that the pulse signalcan be recovered.

FIG. 2 depicts a flow chart 200 of exemplary steps for modulating apulse signal for transmission. The exemplary steps are described withreference to FIG. 1. At block 202, the shift register 110 receives a bitstream. In an exemplary embodiment, the shift register 110 shifts thebit stream through the shift register 110 two bits at a time.

At block 204, the phase controller 112 generates a phase signalresponsive to the bit stream. In an exemplary embodiment, the phasecontroller 112 generates a phase signal responsive to a first bit of thebit stream and every other bit thereafter. In an alternative exemplaryembodiment, the phase controller 112 generates the phase signalresponsive to a second bit of the bit stream and every other bitthereafter. The phase signal may be a binary signal that is set to arelatively high (low) value when a bit is high (i.e., a logical one “1”)and is set to a relatively low (high) value when the bit is low (i.e., alogical zero “0”).

At block 206, the delay controller 114 generates a delay signalresponsive to the bit stream. In an exemplary embodiment, the delaycontroller 114 generates a delay signal responsive to a second bit ofthe bit stream (i.e., the next consecutive bit following the first bit)and every other bit thereafter. In an alternative exemplary embodiment,the delay controller 114 generates the delay signal responsive to thefirst bit of the bit stream and every other bit thereafter. The delaysignal may be a binary signal that is set to a relatively high (low)value when a bit is high and is set to a relatively low (high) valuewhen the bit is low.

At block 208, the pulse generator 104 generates the pulse signalresponsive to the phase signal and the delay signal received from thepulse controller 102. The pulse generator 104 selectively inverts signalpulses responsive to the phase signal and selectively delays signalpulses responsive to the delay signal. In an exemplary embodiment, wherea single pulse generator 192 (see FIG. 1C) is used in the pulsegenerator 104, the pulse generator 104 does not alter the signal pulseif the phase signal and the delay signal are both relatively high (low),delays the signal pulse if the phase signal is relatively low (high) andthe delay signal is relatively high (low), inverts the signal pulse ifthe phase signal is relatively high (low) and the delay signal isrelatively low (high), and inverts and delays the signal pulse if boththe phase signal and the delay signal are relatively high (low).

In an alternative exemplary embodiment, where a plurality of pulsegenerators 152 (see FIG. 1A) are used in the pulse generator 104, theselector 154 selects a first signal pulse (which is not delayed and notinverted) produced by the first pulse generator 156 if the phase signaland the delay signal are both relatively high (low), selects a secondsignal pulse (which is delayed and not inverted) produced by the secondpulse generator 158 if the phase signal is relatively low (high) and thedelay signal is relatively high (low), selects a third signal pulse(which is inverted and not delayed) produced by the third pulsegenerator 160 if the phase signal is relatively high (low) and the delaysignal is relatively low (high), and selects a fourth signal pulse(which is delayed and inverted) produced by the fourth pulse generator162 if both the phase signal and the delay signal are relatively high(low).

In an alternative exemplary embodiment, where a pair of pulse generators172 (see FIG. 1B) are used in the pulse generator 104, the selector 174selects a first signal pulse (which is not inverted) produced by thefirst pulse generator 178 if the phase signal is relatively high (low)and selects a second signal pulse (which is inverted with respect to thefirst signal pulse) produced by the second pulse generator 180 if thephase signal is relatively low (high). The delay circuit 176 thenintroduces delay to the selected signal if the delay signal isrelatively high (low) and does not introduce delay if the delay signalis relatively low (high).

Exemplary pulses are shown in FIGS. 7A, 7B, 7C, and 7D. FIG. 7A depictsa monocycle pulse 710. FIG. 7B depicts an inverted monocycle pulse 712.FIGS. 7C and 7D depict a delayed monocycle pulse 714 and a delayed andinverted monocycle pulse 716, respectively. Because there are fourpossible pulses, each pulse may represent two bits of the bit stream.The pulses may be assigned as shown in Table 1.

TABLE 1 Value (first bit, second bit) Pulse 0, 0 monocycle pulse 0, 1inverted monocycle pulse 1, 0 delayed monocycle pulse 1, 1 delayed andinverted monocycle pulse

Referring back to FIG. 2, at block 210, the optional time-hoppingcontroller 106 time-hops the pulse signal. In an exemplary embodiment,the time-hopping controller 106 places each modulated signal pulse at adifferent time-hop index inside a frame. Each modulated signal pulse maybe repeated inside the frame or across multiple frames in order toprovide redundancy for a multiple-access environment. In an alternativeexemplary embodiment, the pulse signal is not time-hopped and the stepin block 210 can be eliminated.

At block 212, the transmitter 100 transmits the pulse signal via theantenna 108. In an exemplary embodiment, the pulse signal, asselectively inverted, delayed, and time-hopped, is transmitted. In analternative exemplary embodiment, the pulse signal as selectivelyinverted and delayed is transmitted without time-hopping.

Additional details regarding modulation techniques for use with UWBsignals will now be described. UWB signals can be modeled as shown inequation (1).

$\begin{matrix}{{S(t)} = {\sum\limits_{l = {- \infty}}^{\infty}{\frac{1}{N}{\sum\limits_{i = 0}^{N - 1}{A_{l} \cdot {X_{pulse}\left( {t - {i \cdot T_{PPM}} - {l \cdot T_{symbol}}} \right)}}}}}} & (1)\end{matrix}$In equation (1), A_(i) and T_(PPM) represent the data, and T_(symbol)represents the symbol index that is being transmitted. X_(pulse)represents the waveform including pulse shape and transmission power.There are different techniques for transmitting data over an UWBchannel. These methods are now described.

The PSD for the monocycle pulse typically used in UWB communications isshown as plot 300 in FIG. 3. This plot and the other PSD plots depictedin FIGS. 4–8 were generated using a simulation in which each pulse isrepresented by 64 samples and each frame is 64 times longer than asingle pulse, thus, a frame includes 2048 samples. Each simulation wasrun for 3,000 repetitions. The number of points in the fast Fouriertransform (FFT) was 262,144. In each repetition, the data being sent wasrandomized 128 times. The randomization included both bi-phase and pulseposition randomness. All of the PSD plots are generated using theBartlett periodogram method described in a text by J. G. Proakis et al.entitled Digital Signal Processing, Prentice Hall, third edition, 1996.

Pulse position modulation (PPM) is now described. PPM is one of the mostpopular modulation methods used in UWB communication systems. The majoradvantage of PPM is its power efficiency, i.e., as the number of levels(M) increase, there is no corresponding increase in power. The number oflevels indicate the number of modulation positions. For example, forM-PPM, where M=2, two (2) modulation positions are needed; where M=4,four (4) modulation positions are needed. As shown in the followingequations, however, PPM modulation has relatively high power spectrallines in its PSD. Therefore, if data is transmitted using PPMmodulation, the average power per pulse may need to be reduced for thepower spectral density of the pulse to be within emission limitsspecified by the FCC for UWB communications (referred to herein as theFCC mask), which is undesirable.

Equation (2) below, is taken from a textbook by S. Wilson entitledDigital Modulation and Coding Prentice Hall, 1995. This equation is usedto calculate the PSD for the PPM modulation.

$\begin{matrix}\begin{matrix}{S_{{yy} - {ppm}} = {{\frac{1}{M^{2}T_{f}^{2}}{\sum\limits_{n = {- \infty}}^{\infty}{{{\sum\limits_{i}{S_{i}\left( \frac{n}{T_{f}} \right)}}}^{2}{\delta\left( {f - \frac{n}{T_{f}}} \right)}}}} +}} \\{\frac{1}{T_{f}}\left\lbrack {{\sum\limits_{i}{\frac{1}{M}{{S_{i}(f)}}^{2}}} - {{\sum\limits_{i}{\frac{1}{M}{S_{i}(f)}}}}^{2}} \right\rbrack}\end{matrix} & (2)\end{matrix}$

In equation (2), S_(i)(f) is the Fourier transform of the monocyclepulse, T_(f) is the frame time, and M the number of levels. A newparameter may be introduced to form a new equation, e.g., one for 2-PPMmodulation in which each pulse is positioned in one of two positionsduring modulation. The Fourier transform for 2-PPM is given by equation(3).

$\begin{matrix}{{H_{2 - {ppm}}(f)} = {\sum\limits_{k = 0}^{1}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}} & (3)\end{matrix}$In equation (3), T_(p) is the pulse time.Combining equations (2) and (3) yields equation (4).

$\begin{matrix}\begin{matrix}{S_{{yy} - {2\mspace{11mu}{ppm}}} = {{\frac{1}{4\; T_{f}^{2}}{\sum\limits_{n = {- \infty}}^{\infty}{{{\sum\limits_{k = 0}^{1}{{S(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}}^{2}{\delta\left( {f - \frac{n}{T_{f}}} \right)}}}} + \ldots +}} \\{\frac{1}{T_{f}}\left\lbrack {{\sum\limits_{k = 0}^{1}{\frac{1}{2}{{{S(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}^{2}}} -} \right.} \\\left. {{\sum\limits_{k = {- 0}}^{1}{\frac{1}{2}{S(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}}^{2} \right\rbrack \\{= {{\frac{1}{4T_{f}^{2}}{\sum\limits_{n = {- \infty}}^{\infty}{{S^{2}\left( \frac{n}{T_{f}} \right)}{\cos^{2}\left( {\omega\;\frac{T_{p}}{4}} \right)}{\delta\left( {f - \frac{n}{T_{f}}} \right)}}}} +}} \\{\frac{1}{T_{f}}\left\lbrack {{S^{2}(f)} - {\frac{1}{2}{S^{2}(f)}} - {\frac{1}{2}{S^{2}(f)}{\cos^{2}\left( {\omega\;\frac{T_{p}}{4}} \right)}}} \right\rbrack}\end{matrix} & (4)\end{matrix}$

It is noted that equation (4) includes two components: a discretecomponent and a continuous component. The discrete component, whichrepresents the spectral lines encountered in PPM modulation, is shown inequation (5).

$\begin{matrix}{S_{{yy} - {disc}} = {\frac{1}{4T_{f}^{2}}{\sum\limits_{n = {- \infty}}^{\infty}{{S^{2}\left( \frac{n}{T_{f}} \right)}{\cos^{2}\left( {\pi\;\frac{{nT}_{p}}{2T_{f}}} \right)}{\delta\left( {f - \frac{n}{T_{f}}} \right)}}}}} & (5)\end{matrix}$The continuous component, which represents the pulse shape and the pulseposition modulation filter, is shown in equation (6).

$\begin{matrix}{S_{{yy} - {cont}} = {\frac{1}{T_{f}}\left\lbrack {{S^{2}(f)} - {\frac{1}{2}{S^{2}(f)}{\cos^{2}\left( {\omega\;\frac{T_{p}}{4}} \right)}}} \right\rbrack}} & (6)\end{matrix}$The PSD derived from the PPM signal is shown in FIG. 4. FIG. 4 clearlyshows both the power spectrum 400 of the pulse shape and the powerspectrum of the discrete components 410. The spectral lines 410 arerelatively high. Accordingly, the average power per pulse is desirablyreduced to fit the signal within the FCC mask for UWB transmission.

Bi-phase shift keying (BPSK) modulation in now described. With BPSKmodulation the spectral lines associated with the power spectral densityof the signal itself are reduced, which is an advantage over PPMmodulation; however, power efficiency with BPSK modulation decreases forM≧4. BPSK modulation uses a monocycle pulse and its inverse to transmitdata. The monocycle pulse represents one logic state, for example, logicone (“1”) and the inverse pulse represents the other state, for example,logic zero (“0”). FIGS. 5A and 5B show a waveform 510 of the monocyclepulse and the waveform 520 of its inverse, respectively.

The PSD for a BPSK modulated signal is now derived. The data streamtransmitted using the BPSK signal is assumed to be perfectly random and,thus, the BPSK modulation may be represented as shown in equation (7).S _(i)(f)=Σ_(i=0) ¹(2i−1)Φ(f)  (7)In equation (7), Φ(f) represents the pulse shape. Introducing this modelinto the above power spectrum equation, produces equation (8).

$\begin{matrix}\begin{matrix}{S_{yy} = {{\frac{1}{4T_{f}^{2}}{\sum\limits_{n = {- \infty}}^{\infty}{{{\sum\limits_{i = 0}^{1}{\left( {{2i} - 1} \right){\Phi(f)}}}}^{2}{\delta\left( {f - \frac{n}{T_{f}}} \right)}}}} +}} \\{\frac{1}{T_{f}}\left\lbrack {{\frac{1}{2}{\sum\limits_{i = 0}^{1}{{\left( {{2i} - 1} \right){\Phi(f)}}}^{2}}} - {{\sum\limits_{i = 0}^{1}{\left( {{2i} - 1} \right){\Phi(f)}}}}^{2}} \right\rbrack} \\{= {\frac{1}{T_{f}}\left\lbrack {\Phi^{2}(f)} \right\rbrack}}\end{matrix} & (8)\end{matrix}$The power spectrum 600 generated from the simulation using equation (8)is shown in FIG. 6.

As shown in FIG. 6, the PSD of a BPSK modulated random data signal isessentially the same as the PSD of the monocycle signal without anyspectral lines. The difference in power between BPSK and PPM modulationas may be seen from FIGS. 4 and 6 is about 10 dB, which illustrates theadvantage of bi-phase modulation. Moreover, because the PSD isessentially the PSD of the monocycle pulse, different pulse shapes canbe tried in the simulation to identify shapes useful for UWBtransmissions.

The modulation scheme of the present invention is now described. Thisscheme merges PPM and BPSK modulation to produce a modulation schemereferred to herein as biorthogonal modulation. The merging of these twotechniques provides improved power efficiency and reduces/eliminates PSDspectral lines from the pulse shape. In an exemplary embodiment,biorthogonal modulation uses the monocycle pulses shown in FIGS. 7A, 7B,7C and 7D.

Because the data stream is assumed to be perfectly random, thebiorthogonal signal may be modeled as shown in equation (9).

$\begin{matrix}{{S_{i}(f)} = {\sum\limits_{i = 0}^{1}{\sum\limits_{k = 0}^{1}{\left( {{2i} - 1} \right){\Phi(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}}} & (9)\end{matrix}$In this equation, Φ(f) represents the pulse shape.

Introducing this model into the above power spectrum equation (2)produces the power spectrum equation (10).

$\begin{matrix}\begin{matrix}{S_{yy} = {\frac{1}{16T_{f}^{2}}{\sum\limits_{n = {- \infty}}^{\infty}{{\sum\limits_{i = 0}^{1}{\sum\limits_{k = 0}^{1}{\left( {{2i} - 1} \right){\Phi(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}}}^{2}}}} \\{{\delta\left( {f - \frac{n}{T_{f}}} \right)} + \ldots +} \\{{\frac{1}{T_{f}}\left\lbrack {\frac{1}{4}{\sum\limits_{i = 0}^{1}{\sum\limits_{k = 0}^{1}{{\left( {{2i} - 1} \right){\Phi(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}^{2}}}} \right\rbrack} - \ldots -} \\{\frac{1}{T_{f}}\left\lbrack {{\sum\limits_{i = 0}^{1}{\sum\limits_{k = 0}^{1}{\frac{1}{4}\left( {{2i} - 1} \right){\Phi(f)}{{Exp}\left\lbrack {{- j}\;{\omega\left( {\frac{T_{p}}{4} + {\frac{T_{p}}{2}k}} \right)}} \right\rbrack}}}}}^{2} \right\rbrack} \\{= {\frac{1}{T_{f}}\left\lbrack {\Phi^{2}(f)} \right\rbrack}}\end{matrix} & (10)\end{matrix}$

From equation (10), it is noted that biorthogonal modulation providesthe PSD of the monocycle pulse essentially without any spectral lines(which is similar to the bi-phase modulation described above). Thisresults from the random changing of the polarity of the pulse such thateven when used in conjunction with PPM modulation there is nocorrelation between any of the present and future pulses. Therefore,because there is no correlation between pulses, the resulting PSD 800 isshown in FIG. 8. Thus, the PSD for biorthogonal modulation isessentially the same as for a BPSK signal with time-hopping in which thepulse is not repeated (i.e., one in which the polarity of the pulses arerandomly changed over the hops).

This biorthogonal modulation method achieves several important results.By way of non-limiting example, this method provides the following fouradvantages. First, a user may send two bits per symbol instead of onebit as in a PPM or a BPSK modulation scheme. Second, this method mayachieve the power efficiency of a PPM signal because the number oflevels, M_(ppm), may be increased (e.g., by increasing the number ofpossible delays for sending the bi-phase pulse) without increasing theaverage power. Third, this method substantially eliminates spectrallines due to no/low correlation between signal pulses. Fourth, thismethod enables multi-user access. From the simulations, it is seen thatbiorthogonal modulation achieves the same result as bi-phase modulationwith an added feature that a user is sending two bits per symbol insteadof one. Another advantage of biorthogonal modulation is that the powerspectral density of the pulse is achieved. Therefore, one can meet theFCC mask requirement merely by using a pulse that has a PSD which meetsthe mask requirements.

The PSD for biorthogonal modulation is essentially the PSD of themonocycle pulse for biorthogonal modulated signals with time-hoppingsequences when the pulse is not repeated between time-hops. Essentiallyfrom one time-hopping index to the next, a different modulated pulse issent. This means, that no matter what pulse is used, it is expected thatthe PSD will be the PSD of the pulse shape by itself. The inventors havedetermined that the PSD of the monocycle pulse (i.e., the 2nd derivativeof a Gaussian pulse) may not meet the PSD mask requirements imposed bythe FCC. The inventors have also determined that higher derivatives ofthe Gaussian pulse (e.g. 5th order and above) meet these requirements.Because biorthogonal modulation maintains the PSD of the pulse shape,any pulse shape that has a PSD within the FCC mask may be used to meetthe FCC's PSD mask requirements for the biorthogonal modulated signalwithout the need of any other pulse manipulation.

Although the invention has been described in terms of a transmitter 100including a pulse controller 102, pulse generator 104, and time-hoppingcontroller 106, it is contemplated that the invention may be implementedin software on a computer (not shown), such as a general purposecomputer, special purpose computer, digital signal processor,microprocessor, microcontroller, or essentially any device capable ofprocessing signals. In this embodiment, one or more of the functions ofthe various components may be implemented in software that controls thecomputer. This software may be embodied in a computer readable carrier,for example, a magnetic or optical disk, a memory-card or an audiofrequency, radio-frequency, or optical carrier wave.

In addition, although the invention is illustrated and described hereinwith reference to specific embodiments, the invention is not intended tobe limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the invention.

1. An apparatus for modulating a bit stream onto a pulse signal for transmission, the apparatus comprising: a pulse generator that generates a selectively delayed and inverted signal pulse responsive to a phase signal and a delay signal; a phase controller coupled to the pulse generator, the phase controller configured to generate the phase signal responsive to a first data bit of the bit stream; and a delay controller coupled to the pulse generator, the delay controller configured to generate the delay signal responsive to a second data bit of the bit stream.
 2. The apparatus of claim 1, wherein the pulse generator is a monocycle pulse generator and the signal pulse is a monocycle signal pulse generated by the monocycle pulse generator.
 3. The apparatus of claim 1, wherein the pulse generator is an Ultra Wideband (UWB) pulse generator and the signal pulse is a UWB signal pulse.
 4. The apparatus of claim 1, wherein the signal pulse is selectively delayed by an amount sufficient to substantially decorrelate the delayed signal pulse from a non-delayed signal pulse.
 5. The apparatus of claim 4, wherein the delayed signal pulse is orthogonal to the non-delayed signal pulse.
 6. The apparatus of claim 1, further comprising: a time-hopping controller for time-hopping the signal pulse as selectively inverted and delayed.
 7. The apparatus of claim 1, wherein the first and second data bits are consecutive bits that are represented by the signal pulse as selectively delayed and inverted.
 8. The apparatus of claim 1, wherein the pulse generator comprises: a first pulse generator that generates a non-inverted signal pulse with no delay; a second pulse generator that generates a non-inverted signal pulse with a delay; a third pulse generator that generates an inverted signal pulse with no delay; a fourth pulse generator that generates an inverted signal pulse with the delay; and a selector coupled to said first, second, third, and fourth pulse generators, the selector selecting the signal pulse generated by one of the first, second, third, and fourth pulse generators responsive to the phase signal and the delay signal.
 9. The apparatus of claim 1, wherein the pulse generator comprises: a pulse generator that generates a signal pulse, the pulse generator configured to selectively invert the signal pulse responsive to the phase signal and to selectively delay the signal pulse responsive to the delay signal.
 10. The apparatus of claim 1, wherein the pulse generator comprises: a first pulse generator that generates a first pulse; a second pulse generator that generates a second pulse, the second pulse being inverted with respect to the first pulse; a selector coupled to the first and second pulse generator, the selector selecting the first pulse generated by the first pulse generator or the second pulse generated by the second pulse generator responsive to the phase signal; and a delay circuit coupled to the selector, the delay circuit introducing delay to the pulse selected by the selector responsive to the delay signal.
 11. A method for modulating a bit stream onto a pulse signal for transmission, the method comprising the steps of: selectively inverting a signal pulse responsive to a first bit of the bit stream; and selectively delaying the signal pulse responsive to a second bit of the bit stream.
 12. The method of claim 11, wherein the signal pulse is a monocycle signal pulse and wherein the inverting and delaying steps, respectively, comprise the steps of: selectively inverting the monocycle signal pulse responsive to the first bit of the bit stream; and selectively delaying the monocycle signal pulse responsive to the second bit of the bit stream.
 13. The method of claim 12, wherein the monocycle signal pulse is an Ultra Wideband (UWB) signal pulse.
 14. The method of claim 11, wherein the delaying step comprises the step of: selectively delaying the signal pulse responsive to the second bit of the bit stream by an amount sufficient to substantially decorrelate the delayed signal pulse from the original signal pulse.
 15. The method of claim 14, wherein the delaying step comprises the step of: selectively delaying the signal pulse such that the delayed signal pulse is orthogonal to the signal pulse prior to delay.
 16. The method of claim 11, further comprising: time-hopping the signal pulse as selectively inverted and delayed.
 17. The method of claim 11, wherein the inverting step comprises the steps of: generating a phase signal responsive to the first bit; and selectively inverting the signal pulse responsive to the phase signal.
 18. The method of claim 11, wherein the delaying step comprises the steps of: generating a delay signal responsive to the second bit; and selectively delaying the signal pulse responsive to the delay signal.
 19. A method for modulating a bit stream onto a pulse signal for transmission, the method comprising the steps of: receiving a bit stream having a first and a second bit; and selectively inverting and delaying a signal pulse responsive to the first and second bits of the bit stream.
 20. The method of claim 19, wherein the inverting and delaying step comprises the steps of: generating a phase signal and a delay signal responsive the first and second bits of the bit stream; and selectively inverting and delaying the signal pulse responsive to the phase signal and the delay signal.
 21. The method of claim 19, wherein the inverting and delaying step comprises the steps of: generating a non-inverted signal pulse with no delay, a non-inverted signal pulse with delay, an inverted signal pulse without delay, and an inverted signal pulse with delay; and selecting one of the generated signal pulses responsive to the first and second bits of the bit stream.
 22. The method of claim 21, wherein the selecting step comprises the steps of: generating a phase signal and a delay signal responsive the first and second bits; and selecting one of the generated signal pulses responsive to the phase signal and the delay signal.
 23. The method of claim 19, wherein the inverting and delaying step comprises the steps of: generating a non-inverted signal pulse and an inverted signal pulse; selecting one of the generated signal pulses responsive to the first bit of the bit stream; and selectively delaying the selected one of the generated signal pulses responsive to the second bit of the bit stream.
 24. A system for modulating a bit stream onto a pulse signal for transmission, the system comprising: means for selectively inverting a signal pulse responsive to a first bit of the bit stream; and means for selectively delaying the signal pulse responsive to a second bit of the bit stream.
 25. The system of claim 24, further comprising: means for time-hopping the signal pulse as selectively inverted and delayed.
 26. The system of claim 24, wherein the inverting means comprises: means for generating a phase signal responsive to the first bit; and means for selectively inverting the signal pulse responsive to the phase signal.
 27. The system of claim 24, wherein the delaying means comprises: means for generating a delay signal responsive to the second bit; and means for selectively delaying the signal pulse responsive to the delay signal.
 28. A computer readable carrier including software that is configured to control a computer to implement a modulation method embodied in a computer readable medium, the modulation method including the steps of: selectively inverting a signal pulse responsive to a first bit of the bit stream; and selectively delaying the signal pulse responsive to a second bit of the bit stream.
 29. The computer readable carrier of claim 28, wherein the inverting step for implementation by the computer comprises the steps of: generating a phase signal responsive to the first bit; and selectively inverting the signal pulse responsive to the phase signal.
 30. The computer readable carrier of claim 28, wherein the delaying step for implementation by the computer comprises the steps of: generating a delay signal responsive to the second bit; and selectively delaying the signal pulse responsive to the delay signal.
 31. The computer readable carrier of claim 28, wherein the method implemented by the general purpose computer further includes the step of: time-hopping the selectively inverted and delayed signal pulse. 